In a dry combustion environment where natural gas or the like is combusted, since combustion residuals do not readily form on the surface of an engine valve, direct metal-metal contact occurs between a valve seat surface and a seat insert, so that the valve seat suffers severe adhesive wearing. In this connection, Tribaloy T400 (Registered Trademark), a cobalt base abrasion resistance alloy, has been used in natural gas and other such environments where severe abrasion occurs. However, since material cost is high and workability is poor in the case of using the cobalt base abrasion resistance alloy, this is fatal flaw in manufacturing a valve.
Various proposals have been made for reducing the cost of engine valves that have high durability in a dry combustion environment. In particular, Patent Publication #1 indicated below proposes to form a surface-hardened layer by effecting nitriding treatment on a seat weld metal of an iron-base alloy of a specified composition, thereby increasing the durability of an engine valve at a low cost.